N.W.T. Diamonds — BHP-Dia Met age-date kimberlites

The age of two Lac de Gras kimberlites has been disclosed by joint-venture partners BHP Minerals Canada and Dia Met Minerals (TSE). The partners were the first to discover potentially economic diamondiferous kimberlites in the Lac de Gras region of the Northwest Territories.

Rory Moore, BHP Minerals Canada’s manager of diamond exploration, said consistent results were obtained from two completely independent approaches, namely radiogenic isotopic measurements and paleontological studies. Moore said rubidium-strontium systematics were used to obtain an isochron age of 52 1.2 million years (95% confidence level) for one pipe. The isochron was defined by two mica separates and a whole rock sample. The work was undertaken in the Radiogenic Isotope Facility of the University of Cape Town, under the supervision of Dr. John Gurney.

The paleontological work was carried out by Dr. W.W. Nassichuk and D.J. McIntyre of the Geological Survey of Canada in Calgary, who recovered dinoflagellates, pollen, spores and teleost fish remains from mudstones incorporated in two kimberlite pipes.

The fossils range in age from Early Cretaceous (Albian) to early Tertiary (Paleocene) — that is, from 97-56 million years ago. Accordingly, paleontological data closely support the rubidium-strontium age of emplacement for the kimberlites.

The joint venture intends to report these results in a scientific journal in the near future.

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